Conventionally, an emission treatment system has been disclosed which is suitable for treating an exhaust stream containing NOx and particulate matter (see patent document 1, for example). This emission treatment system includes an oxidation catalyst; an injector in fluid communication with and downstream of the oxidation catalyst, wherein the injector periodically meters ammonia or an ammonia precursor into the exhaust stream; and a wall flow monolith in fluid communication with and downstream of the injector. The wall flow monolith has a plurality of longitudinally extending passages formed by longitudinally extending walls bounding and defining the passages. Here, the wall flow monolith comprises an SCR catalyst composition that permeates the walls at a concentration of at least 1.3 g/in3, and the wall flow monolith has a wall porosity of at least 50% with an average pore size of at least 5 μm. Further, the SCR catalyst composition comprises: a base metal component selected from one or more of copper and iron components; and a zeolite; and the zeolite has a silica to alumina ratio of at least about 10. This SCR catalyst composition is deposited onto inlet passages and outlet passages of the wall flow monolith, and then dried and fired.
In the thus constituted emission treatment system, the SCR catalyst composition is capable of effectively catalyzing the reduction of NOx components at temperatures lower than 600° C., and is also capable of aiding in the regeneration of the monolith acting as a filter by lowering the temperature at which the soot fraction of the particulate matter is combusted. Further, the SCR catalyst composition catalyzes a reaction of O2 with somewhat excessively present NH3 to generate N2 and H2O so that NH3 is not emitted into the atmosphere, and the SCR catalyst composition is configured to withstand temperatures exceeding 650° C. Here, those zeolites, which are highly resistant to sulfate poisoning and provide good activity for both the SCR process and the oxidation of ammonia with oxygen, and which retain good activity even when subject to high temperatures, hydrothermal conditions and sulfate poisons, are zeolites having pores which exhibit a pore diameter of at least about 7 angstroms and which are interconnected in three dimensions. It is said to be believed that at the interconnection of pores of at least 7 angstroms diameter in three dimensions provides for good mobility of sulfate molecules throughout the zeolite structure, thereby permitting the sulfate molecules to be released from the catalyst to free a large number of the available adsorbent sites for reactant NOx and NH3 molecules and reactant NH3 and O2 molecules. Note that examples of zeolites satisfying such criteria include USY zeolite (ultrastable Y-type zeolite), beta zeolite (alumino silicate comprising pores defined by three-dimensionally twelve-membered rings), and the like.
Patent document 1: Japanese Patent Application Laid-Open Publication No. 2007-501353 (claims 1, 5, 6, and 12; paragraphs [0035], [0036], and [0038])